The present invention relates to endpoint detection in a polishing process, and more particularly to endpoint detection based on friction between the polishing tool and the structure being polished.
Chemical mechanical polishing (CMP) is widely used in fabrication of integrated circuits. FIGS. 1 and 2 illustrate fabrication of tungsten plugs 120 that provide electrical contact between a layer 130 and another, overlying layer (not shown). Layer 130 can be a metal layer (e.g. tungsten) formed over a monocrystalline silicon substrate 140 and, possibly, over some other layer or layers 150. Silicon dioxide 160 is formed over layer 130. Openings 170 are etched in oxide 160 to expose metal 130. A thin titanium nitride layer 110 is deposited (e.g. sputtered) over the structure to promote adhesion (tungsten 120 does not adhere well to silicon dioxide). Then tungsten 120 is deposited by chemical vapor deposition to fill the openings 170 and cover the structure. The top surface of the structure is polished by CMP until tungsten 120 and titanium nitride 110 are removed from the top surface. The resulting structure is shown in FIG. 2. Another conductive layer (not shown) can be formed on this structure. This layer will electrically contact the layer 130 through the metal plugs 120/110 in openings 170. (For brevity, we will refer to plugs 120/110 as tungsten plugs 120.)
The CMP process should remove all of the tungsten 120 and titanium nitride 110 from the top surface of oxide 160 in order to avoid electrical shorts and excessive current leakage between the plugs. The CMP endpoint can be determined by monitoring the friction between the wafer and a polishing pad of the CMP tool. FIG. 3 illustrates an example CMP tool available from SpeedFam-IPEC of Chandler, Ariz. Wafer 180, which incorporates the structure of FIG. 1, is held upside down on a carrier 210 (the wafer""s front side 180F faces down). A motor (not shown) rotates the carrier 210, thus causing the wafer to rotate. Another motor 212 rotates a polishing pad 220 which polishes the wafer. (The motor rotates a platen on the pad is positioned). Friction sensor 230 detects the friction between pad 220 and wafer 180 by detecting the current drawn by motor 212. Controller 240 stops the polishing process based on the friction data from sensor 230. Suitable controllers 240 and sensors 230 are available from LUXTRON Corporation of Santa Clara, Calif.
FIG. 4 is a chart showing the friction data FR produced by sensor 230. FR is shown as a function of time. Initially, signal FR decreases as tungsten 120 and titanium nitride 110 are being polished. At some time t1, signal FR levels off, indicating that the polishing pad has reached the oxide 160. Software programmable controller 240 (FIG. 3) is programmed to stop the CMP when the signal FR levels off.
FIG. 5 shows signal FR for another CMP process. In this example, layer 110 is titanium. At some time t2, the friction FR starts to rise. Then FR falls (starting at some time t3), and then levels off at a time t1 when the oxide 160 is reached. Controller 240 is programmed to perform the following steps, in the order shown:
1. Detect rising friction.
2. Detect falling friction.
3. Detect the friction leveling off, and declare an endpoint to stop the CMP.
The inventors have observed that the endpoint detection method described above (stopping the CMP when FR levels off) results in excessive over-polishing. Too much of oxide 160 gets polished off. In some embodiments of the present invention, the CMP is stopped before FR levels off. In the example of FIG. 6, the friction signal FR is as in FIG. 5. The CMP is stopped a predetermined time xe2x80x9cdtxe2x80x9d after the time t3 (the time when FR starts falling off). Controller 240 is programmed to:
1. Detect rising friction.
2. Detect when the friction starts to fall.
3. Declare an endpoint the predetermined time dt after the friction starts to fall.
Step 1 (detect rising friction) assumes that at some point of time the signal FR is rising. In FIG. 4 (the titanium nitride case), the signal FR does not rise. In some embodiments, the titanium nitride deposition parameters are chosen so that the friction signal FR rises at some point (as in FIG. 5).
Other features of the invention are described below. The invention is defined by the appended claims.